Telecommunication system



J. R. V. OSWALD TELECOMMUNICATION SYSTEM Original Filed March 1, 1952Feb. 28, 1961 2 Sheets-Sheet 1 NAZJYLAIH Invenror:

Feb. 28, 1961 J. R. v. OSWALD TELECOMMUNICATION SYSTEM Original FiledMarch 1, 1952 2 Sheets-Sheet 2 Q98. 4mm

w M v slo 9% V J Wiw 'IELECOMMUNICATION SYSTEM Jacques Regis VictorOswald, Paris, France, assignor to Compagnie Industrielle desTelephones, Paris, France, a corporation of France 1 Claim. (Cl. 179-15)The present invention relates to a method of modulation and demodulationfor pulse-type telecommunication system, employing the so-calledposition or time shift modulation and particularly for multiplexsystems. This application is a divisional application of US. applicationSerial No. 274,373 filed March 1, 1952, now Pat. #2,785,227 issued March12, 1957.

The principle employed in multiplex systems consists, as is well-known,in transmitting a series of impulses corresponding to the differentchannels, in the order of said channels, a special synchronising impulsefixing the period of repetition. The position modulation has the effectof causing a displacement of the impulse in relation to its referenceposition, which displacement is in proportion to the amplitude of thesignal to be transmitted.

In the majority of known modulation systems, positionmodulated impulsesare obtained by differentiating duration-modulated impulses, for exampleby means of a condenser and a resistance. The duration-modulation isitself obtained from an amplitude-modulated impulse, which undergoes asuitable deformation by means of a time-constant device.

The method of modulation according to the invention makes it possible toobtain position-modulated impulses from amplitude-modulated impulses,without passing through the intermediate stage of time modulation.

The object of the present invention is a method of position modulationfor a pulse-type telecommunication system, characterised in this, thatthe position modulation is obtained directly from an amplitudemodulation, by addition to a first series of fixed impulses representingthe original impulses before modulation, of a second series of impulsesobtained by effecting the differentiation, in relation to time, of thefirst series of impulses after they have been amplitude-modulated.

The demodulation is obtained by addition to the series ofposition-modulated impulses of a series of fixed impulses of oppositepolarity, and by integration, in relation to time, of the signalsobtained in this way.

The principle employed is the result of a mathematical expressionindicated hereinafter:

Let an electrical impulse be of any form, capable of being representedby a continuous function of time f(t) which has a differentialcoeflicientfor all values of t. Any series of 2p+1 identical andequidistant impulses may be written: 7

designating by the t =kT the characteristic time unit of the kth impulseand by T the period of repetition.

Let x(l) be any signal representing the modulation to be transmitted; inthe time unit t x(1) has the value x =x ([1 i lowing series tocorrespond to y(t):

Patented Feb. 28, 1961 2 In position-modulation, it is thecharacteristic timeunit t =kT which is modified in proportion to x andbecomes:

t' t -l-hTx =(k+hx )T k designating a constant factor, which may becalled rate of modulation and which characterises the displacement towhich the impulse is subjected. We then have the series:

In practice, in nearly all cases the maximum displacement undergone bythe impulse hTx is very slight in relation to the period of repetition T(for example 0.01T or 0.02T).

If we develop z(t) in Taylor series, only keeping the terms of the firstorder we get:

The result may be expressed in this way:

A series of position-modulated impulses, with a displacement which isslight in relation to the period of repetition, may be considered as thesum of two terms: the first term represents the series of unmodulatedimpulses, the second is in proportion to the derivative in relation totime of the series of the same impulses after they have been modulatedin amplitude.

From this result it is possible to infer the method of modulation anddemodulation which is characteristic of the invention:

For the modulation, a series of fixed impulses is used, which take twopaths. In the first they are modulated in amplitude, then differentiatedin relation'to time. In the second path they are unchanged. The ends ofthe two paths are interconnected so as to bring about the superpositionof the two series of signals, which supplies a series ofposition-modulated impulses.

' For the demodulation, the reverse procedure is adopted: the series ofposition-modulated impulses is mixed with a series of fixed impulses,obtained from the synchronis- 7 ing impulses, the polarities beingopposite. Then the. series of impulses obtained is integrated,considered as a function of time, which supplies a series modulatedeither in amplitude or in duration. In either case, the restora tion ofthe modulation signal is effected, as is well known, by a simplefiltering by a low pass filter.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appendedclaims. Theinvention itself, however, both as to its construction andits method ofoperation, together with additional objects and advantages thereof, willbe 7 best understood from the following description of specificembodiments when read in connection with the accom panying drawings, inwhich:

' 'Fig. 1, including Sub-Figures 1.1.- to 1.5, represents thetransformations undergone of modulation;

Fig. 2, including Sub-Figures 2.1 to 2.4, representsprocess ofdemodulation; and

Fig. 3 is anelectrical schematic diagram of a multiplex system using themodulation and demodulation operations of the present invention.

by the impulses in the process Referring to the figures, and moreparticularly to Fig. 1, Fig. 1.1 represents the low frequency signal tobe transmitted. Fig. 1.2 represents the series of unmodulated impulses,separated by the interval'T. 'The product of the impulses of'Fig."1.2with the signal of Fig. 1.1 supplies the amplitude modulated impulsesshown in Fig. 1.3."

When the amplitude modulated impulses of Fig. 1.3 are dififerentiatedwith respect to time, the series of impulses shown in Fig. 1.4 resultsin the impulse series shown in Fig. 1.5. It is apparent that theimpulses in Fig. 1.5 are modulated in position with respect to theimpulses of Fig. 1.2. In Fig. 1.5 a parasitic amplitude modulation ispresent which may be easily eliminated by the use of suitable limitingdevices.

Referring now to Fig. 2 the process of demodulation will be described.In Fig. 2.1 the positive impulses are the fixed reference impulses whichare generated from the synchronizing impulses. The negative impulsesshown in Fig.2.l are the position modulated channel impulses. The sum ofthese negative and positive impulses provide the series of impulsesshown in Fig. 2.2.

The integration of the series of impulses of Fig. 2.2

supplies the series 2.3 which is modulated in amplitude and in duration.It is clear that by limiting or by suitable selection of the impulses itis possible to obtain impulses which are modulated only in duration oronly in amplitudes. Such an operation, however, is unnecessary becausethe spectrum of the modulation signal is present in the series ofimpulses in Fig. 2.3 so that a low frequency signal, such as shown inFig. 2.4 can be obtained merely by using a low pass filter.

It should be noted that the series of impulses shown in Fig. 2.2 isalready duration modulated. However, the integration of these impulsesmakes it possible to disregard, to a certain extent, the relativeposition of the two series of impulses shown in Fig. 2.1. Where thepositive and negative impulses do not interfere, the inte grationsupplies impulses modulated in duration, while direct demodulation ofthe series of impulses in Fig. 2.2 is not possible without considerabledistortion. In addition, the demodulation process described is exactlyreciprocable to the modulation processes.

One of the advantages of the method of modulation and demodulationcarried out by the present invention is that it is not dependent on thecharacteristics of electronic tubes. In effect, the operations ofdifferentiation and integration are efiected solely by means of passivenetworks, comprising inductances, capacitances and resistances. It isnot necessary to' use tubes having a welldefined polarization as is thecase in the majority of the known arrangements.

Referring now to Fig. 3, the portion at in discontinuous lines,represents the members belonging to one channel of the multiplex system,while the portion outside this dotted line represents the members commonto all of the channels. In this figure 1 represents a generator ofsinusoidal oscillations from which an impulse generator 2 supplies aseries of impulses of repetition period T. This series energizes on theone hand a generator 3 of pilot synchronising impulses, on the otherhand a delay line 4 from the taps of which the impulses allotted to thevarious channels are taken. The channel impulse is applied through atransformer 7 toa balanced channel modulator 8 including for example,rectifiers and supplying an amplitudemodulation without carrier wave.

The audio frequency signals (Fig. 1.1.) coming-from a line 6 are appliedto the input transformer 5 of the modulation; The mixture of impulsessupplied by the various channels is attested onthe control, grid of apcntode 11. The product of these impulses, (Fig. 1.3)

is difierentiated by means of a short-circuited delay line 12 (or bymeans of a suitable capacity-resistance system). A tube 13 receives allof the differentiated impulses (Fig. 1.4) on the one hand, and all ofthe fixed impulses coming from the delay line 4 on the other hand; thecoupling being eifected through mixer resistances 14. A limiter tube 15limits the impulses obtained which are then mixed with the synchronisingimpulse from synchronising gen erator 3 on'the output of thetransmittingdevice.

At the receiving end, the received impulses are transmitted withsuitable polarity, by means of transformer 16 to the control grid of atube 17. A selector 18 isolates the synchronising impulse which is sentthrough a delay line 19 to produce the fixed channel selection impulses.These impulses are also mixed through resistances 20 and then broughtback if necessary to the suitable amplitude by a limiter orpotentiometer device 21 before being applied to the control grid of thetube 17. An open circuited delay line 22 (or any other device withcapacity-resistance or inductance-resistance) integrates the signalssupplied by the tube 17 and an amplifier 23 applies,'by means of atransformer 24, the integrated signals to the diflerentjchannels placedin parallel. A suitable selector 25 (for example a blocking selectorwith rectifiers) selects the channel impulse when all of the integratedsignals are applied to it via a transformer 26 and the selection impulsecorresponding to the channel is applied to a resistance 27 from thedelay line 19. The selected impulse is demodulated by a low-pass filter28 and the audio signals amplified by a tube 29 are applied to thetermination 9 of the two-wire line 6.

I will be understood that each of the elements described above, or twoor there together, may also find a useful application in other types ofmodulation differing from the types described abojve.

While the invention has been illustrated and described as embodied in amethod and apparatus for modulation and demodulation for pulse-typetelecommunication, it is not intended to be limited to the detailsshown, since various modifications and structural changes may be madewithout departing in anyway from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist'ofthe present invention that others can by applying current knowledge,readily adapte it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptions should and are intended'to becomprehended within the meaning and range of equivalence. of thefollowing claims.

What is claimed as new and desired to be secured by Letters Patent isz Apulse-type telecommunication system involving a plurality oftransmitting and receiving channels, comprising, in comibnation, anelectrical current impulse gen erator for generating, at predeterminedtime intervals, electrical current impulses capable of being representedby a continuous function-of time, which has a differential coefficientfor all values of time; a modulator arranged in each transmitter channeland having first, second, and output terminal means, said modulatorbeing electrically connected at said first terminal means thereof to theoutput of said electrical current impulse generator, and adapted tobe-connectedat said second terminal means thereof toa source of voicecurrents so that said current ir'npulsesv generated by said currentimpulse generator maybe amplitude modulated by said voice currents; adifferentiatorcircuit common to all transmitting channels andelectricallyconnected to said output terminal means of saidjmodulatorfor differentiating the amplitude modulated current impulses; a'mixercircuit common to all transmitting channels, said mixer having input andoutput terminals and being electrically connected at its input terminalsto the output of said current impulse generator and to the output ofsaid difierentiator circuit for superimposing the two outputs whereby aseries of position modulated electrical current impulses are obtainedfor transmission to a suitable receiving device; a generator in each ofsaid transmission channels and being electrically connected to theoutput of said electrical current impulse generator for generatingelectrical current pilot synchronizing pulses, the latter pulses beingtransmitted together with said position modulated electrical currentimpulses; means for transmitting said position modulated electricalcurrent impulses together with said electrical current pilotsynchronizing pulses; and a receiver for demodulating said transmittedpulses to obtain the desired voice frequencies.

References Cited in the file of this patent UNITED STATES PATENTS

